Revamps 2025 Issue

which can be in a stacked or a side-by-side configuration. These distributors also require the 30% of the bed pressure drop criteria to avoid catalyst entry into the air distributor. Metallurgy has been changed to stainless steel to allow higher bed temperatures and full CO burn in the regenerator. Erosion can occur faster at the higher temperatures, so circu - lating catalyst through any distributor needs to be avoided. Air blowers When more feed is processed, the required coke make will increase. This usually requires more oxygen to process the extra coke produced. Assuming the air blower is at a limit, an auxiliary blower can be added. This is normally the cheapest way to add more oxygen. However, the air distributor might need revamping due to the increased velocity and higher superficial velocity. The latter will increase cyclone loadings while the former might cause catalyst attrition. A revamp of the existing air blower may be possible depending on its type. Centrifugal machines can be rewheeled, or the driver can be changed if more power is required. If the pressure needs to be raised to accommodate the additional traffic through the fractionator, then the pressure balance needs to be checked to handle the increased pressure drops and changes in unit yields. Oxygen enrichment Adding oxygen to the air is another way to increase capac - ity and is very effective when the superficial velocity is 3.3 ft/sec or higher. This reduces the coke make and CO₂ emis - sions, which helps minimise greenhouse emissions. Any unit operating with a hydrotreated feed is a poten - tial candidate for oxygen (O₂) enrichment. These are low delta coke feeds that produce low bed temperatures. With a lower air nitrogen content, more heat is transferred to the catalyst, and the bed temperature goes up about 10ºF for every 1.0% increase in O₂. More optimal burning may occur, allowing a reduction in burning promoter usage. If a CO₂ tax is passed, the reduced coke would provide an additional payback. Depending on the tax rate, oxygen can be added to the air to further reduce the coke needed to operate the process. A higher regenerator temperature also results in a more favourable regeneration operation, since afterburn is lower at higher bed temperatures. CO promoter use might also be reduced. Coke make of around 4.0 wt% would result, and the combination of coke and dry gas should be 7.0 wt% or less (the ‘Letzsch num - ber’) for modern FCCs running with available advanced technologies. If the dry gas is separated into C1s and C2s, the latter can be sent to an ethylene unit and almost quan - titatively converted into ethylene. This reduces the dry gas by about 50-60%. Methane can be used as fuel gas, feed to the hydrogen plant, or converted to electricity. Coke plus dry gas could be less than 6 wt% based on feed. Gas plant If there is a high concentration of diesel in the bottoms stream from the main fractionator, a loss of revenue occurs if this stream is sold as slurry oil. Sometimes, the main frac - tionator bottoms temperature has to be reduced to prevent

coking in the slurry circuit. Some diesel can end up in the bottoms. Another possibility is that the seal oil (LCO) used for the slurry pumps is also lost in the bottoms. A downstream stripper on the bottoms stream can recover this diesel and be both economical and provide more operating flexibility. There may be some modifications to the gas plant that will provide additional value. More cooling of the main column overhead stream, changing trays in part or all of the main col - umn, or changing to packings that reduce the pressure drop are all ways that gas plants have been successfully modified. Better recovery of the liquefied petroleum gas (LPG) might be obtained by cooling a stream to the primary absorber. There are many tweaks that can be made to the gas plant, and a review by experts can identify potential projects. The FCCU will remain a critical part of refining/petrochemical facilities, and it will pay to provide the catalytic cracker with the latest equipment and make it as nimble as possible Catalyst and additive adders The benefits of quasi-continuous catalyst additions are well known. Catalyst deactivation is minimised, and the yields are better. Catalyst losses are lower. A reliable loading sys - tem should be part of every unit. The FCCU should also have a continuous additive load - ing system that allows the unit to load additives as needed. Systems that can add as many as four products at a time are desirable. CO promoter and DeSOx additives might be added regularly, and the use of ZSM-5 and a bottoms cracking additive might be used on an as-needed basis. If ecat is used, a separate silo and loader would give the best performance. Conclusion FCC processing is going to change in the future. Gasoline and diesel demand will decline with more electric vehicles and the integration of refining with petrochemicals. More downstream chemical processes may become part of the processing complex. FCCs will be run at higher tempera - tures that can make both ethylene and propylene, which are fundamental building blocks for petrochemicals. The FCCU will remain a critical part of refining/petrochemical facilities, and it will pay to provide the catalytic cracker with the latest equipment and make it as nimble as possible. Warren S Letzsch has 56 years of experience in petroleum refining, including petroleum catalysts, refining and engineering, and design. He has authored more than 100 technical papers and publications and holds eight patents in the field of FCC. He holds BS and MS degrees in chemical engineering from the Illinois Institute of Technology. Letzsch is a Fellow of the American Institute of Chemical Engineers. Email: wletzsch@verizon.net

Revamps 2025